Diortho Substituted Phenoxyacetic Acids as Antiauxins.

This paper concerns a new class of antiauxins, substances in themselves devoid of activity in the Avena coleoptile section growth test but capable of antagonizing the growth promoting effects of both native and synthetic auxins. These compounds, to which antiauxin activity has not previously been ascribed, are the diortho substituted phenoxyacetic acids. To establish rigorously that a growth inhibiting agent acts by antagonizing the action of auxins, rather than in some other manner, it is necessary to apply a kinetic analysis such as that of LINEWEAVER and BURK (2). This treatment was originally designed to apply to the study of isolated enzyme systems. It will be shown in the present paper, however, that the growth evoked by the application of auxin to a particular plant tissue, the Avena coleoptile section, can be described and treated by the methods of enzyme kinetics. With the aid of this treatment it will be shown that the diortho substituted phenoxyacetic acids competitively and reversibly inhibit the action of indoleacetic acid (IAA) and of 2,4-dichlorophenoxyacetic acid (2,4-D). Avena seedlings (var. Siegeshafer) were grown in vermiculite contained in stainless steel flats and were incubated under red light (with occasional orange light) at 25 to 260 C for 82 to 84 hours. At this time those plants with coleoptiles 2.75 to 3.25 cm. in length were selected for use. One section 5.0 mm. in length, taken 2 to 3 mm. from the tip, was cut with a double bladed cutting tool from each hollow coleoptile cylinder. Each treatment was carried out with 20 such sections, removed after randomization of the entire lot, and floated in Petri dishes containing 20 ml. of solution. The basic medium consisted of Pyrex redistilled water, 3% sucrose and 0.0025 M potassium maleate buffer, pH 4.5. The sections were incubated in the dark at 25 to 260 C. Growth under these conditions is linear with time for approximately 18 to 24 hours. The measurements recorded in this paper were made after incubation for 12 hours and are, therefore, measurements of initial growth velocity. The standard error for the growth of 20 sections within one treatment was on the average 2.1% of the mean. Duplicate means based on 20 sections differed from one another by an average of 3.1%. The influence of varying concentrations of IAA and of 2,6-dichlorophenoxyacetic acid (2,6-DCPA) alone and in combination, on the growth of Avena coleoptile sections is shown in table I. It is clear that 2,6-DCPA is itself inactive over the concentration range tested. It is also apparent that 2,6-DCPA inhibits the section growth induced by IAA and that this inhibition increases with increasing concentrations of 2,6-DCPA. The inhibition